Page 103 - Nanotechnology an introduction

typically triggered by the binding or dissociation of a small molecule; for example the “substrate” of an enzyme, or adenosine triphosphate (ATP).
The initial collision of two particles is followed by a conformational change in one or both of them; e.g.,

(8.20)
where the asterisk denotes a changed conformation induced by binding to A; C has no affinity for B, but binds to B*. This process is illustrated in
Figure 8.6 and is called programmable self-assembly (PSA). Graph grammar, which can be thought of as a set of rules encapsulating the
outcomes of interactions between the particles [93] and [94] (cf. stigmergic assembly, Section 8.2.12), is useful for representing the process. The
concept of graph grammar has brought a significant advance in the formalization of programmable self-assembly, including the specification of
minimal properties that must be possessed by a self-assembling system (e.g., the result implying that no binary grammar can generate a unique
stable assembly [95]).

Figure 8.6 Illustration of programmable self-assembly, with a primitive local rule.
While models of programmably self-assembling robots have been created in the macroscale, artificially synthesizing molecules with the required
attribute remains a challenge. Biology, however, is full of examples (e.g., the “induced fit” occurring when an antibody binds an antigen).
Microscopically, these are manifestations of cooperativity (Section 3.6). However, the cooperativity is systemic in the sense that entire
macromolecules may be acting in concert as a system (cf. 8.2.10 and 8.2.11).
8.2.9. Superspheres

If the competing interactions have different sign and range, ordered structures of definite size can assemble spontaneously. This provides a simple
example of programmable self-assembly. Consider nanoparticles suspended in water and weakly ionized such that they all carry the same
electrostatic charge. When the suspension is stirred, suppose that the repulsive electrostatic force is too weak to overcome the attractive Lifshitz–
van der Waals (LW) force when two particles happen to collide. Therefore, every collision will lead to sticking, and aggregates will slowly form. The
LW force is, however, very short range and can only act between nearest neighbors. The electrostatic force, on the other hand, has a much longer
range, and can therefore be summed over the entire aggregate. Ultimately the aggregate will become large enough for the summed electrostatic
repulsion to exceed the LW nearest neighbor attraction. The result is monodisperse “superspheres” (i.e., aggregates of small (maybe spherical)
particles).

Weakly electrostatically charged quantum dots (nanoparticles) suspended in water aggregate to form uniformly sized superspheres containing
several hundred nanoparticles. Nearest neighbors interact with weak, short range LW interactions, which easily dominate the slight electrostatic
repulsion between them. Because, however, the electrostatic interaction is long range (it can be tuned by varying the ionic strength of the aqueous
solution), the overall electrostatic repulsion within a supersphere gradually accumulates, and when a certain number of nanoparticles have been
aggregated, the electrostatic repulsion exceeds the attractive LW force between nearest neighbors [136]. To form superspheres, the attractive
interaction should be short range, and the repulsive interaction should be long range.
An interesting kind of nanostructure was shown in Figure 6.6(d). The small spheres (called micelles) have polar heads ionizable in water, resulting
in q elementary charges on the surface of the sphere, which exert an expanding pressure

(8.21)
the size of the micelle adjusts itself to exactly compensate the Laplace contraction (equation 2.2); in consequence such micelles are highly
monodisperse because they are at a local energy minimum.

8.2.10. Biological Self-Assembly

It has long been known that many biological systems exhibit remarkable capabilities of assembling themselves starting from a randomly arranged
mixture of components. These include the bacteriophage virus (the final stages of assembly), and proteins and ribonucleic acids (RNA), which can
be spontaneously transformed from a random coil of the as-synthesized linear polymer to a compact, ordered three-dimensional structure (Section
8.2.11). It is clear that the starting precursors of the final structures have to be very carefully designed—this is a carefully tuned example of
programmable self-assembly (PSA) in action (Section 8.2.8).
Although appreciation of self-assembly in biology has played a hugely important inspirational role, the highly specialized chemistry of living
systems, the fragility of many of its products, and its inherent variability at many levels have made it unsuitable for mimicking directly and
incorporating into our present industrial system (cf. Section 8.2.13). This is particularly so in the case of the food industry. The extreme complexity,
both structural and chemical, of its products and the relative ease of letting them grow renders efforts to manufacture food synthetically largely
superfluous.